1. Field of the Invention
Polyether polyurethane moldings are being increasingly used in the manufacture of automobiles and furniture and in home construction. Molded polyether polyurethane foams are especially important because they are lightweight and are resistant to moisture, weather, temperature extremes, and aging. As an illustration, molded, foamed polyether polyurethane elastomers have become of special interest in the manufacture of force-reducing impact media such as safety impact bumpers for automotive vehicles and impact resistant automotive fascia.
The high demand for molded polyether polyurethane articles requires that they be produced in the largest numbers in the shortest possible time. Polyurethane-forming mixtures are eminently suited for mass production because the reactants are liquid, that is they are pumpable, and are quick-reacting. The problem has existed, however, in providing adequate mold release in the shortest possible time to take fullest advantage of the unique capabilities of the polyurethane systems.
As an illustration, it has been proposed to manufacture automotive bumpers and fascia by pumping a polyether polyol and isocyanate prepolymer through a Krauss-Maffei mixing head where intimate mixing of the polyurethane components will occur in a matter of seconds. The mixed, uncured mixture will then proceed, under high pressure, to a mold cavity where the mixture will cure to a solid or microcellular elastomer. The cured polyurethane will be removed in short order to provide a clean mold capable of accepting additional mixture. Cycle times being discussed (from initial filling of the mold to ejection of the cured polyurethane) are in the order of two to three minutes. To achieve these production cycle times, the utilization of an internal release agent in the urethane ingredients is essential.
The field of this invention relates to the quick release of polyurethane compositions from molds in which they are formed and shaped.
2. Description of the Prior Art
Heretofore, release of molded articles from molds in which they have been formed has been achieved by coating the surface of the mold cavity with an agent which facilitates release of the molded article from the walls of the mold cavity. Procedures such as this are described in U.S. Pat. Nos. 3,694,530, 3,640,769, 3,624,190, 3,607,397 and 3,413,390. This procedure has severe drawbacks. The agent, after molding, adheres either to the walls of the mold cavity or to the surface of the molded article or, in the usual case, to both. After multiple moldings and application of release agent, the agent tends to build up on the surface of the mold cavity walls and eventually covers and obscures any detail on the mold cavity surface desired to be imparted to the molded article. Also, the presence of release agent adhering to the surface of the molded article can impede subsequent operations on the article, such as painting or adhering operations. While it is possible to clean the surfaces of molded articles in preparation for painting or adhering operations, this adds to the time and expense of production. Additionally, the need to reapply the release agent after each molding or a limited number of moldings interrupts the molding operation and slows down output.
As an illustration, current practice in molding urethanes is to apply an externally applied substance to the mold cavity. The externally applied release agents are applied by spray or wipe-on techniques. Generally, the release agent adheres to the surface of the molded urethane, thus removing the deposit applied to the mold surface. This presents problems in that additional release agent has to be applied to the mold, and the surface of the urethane has to be degreased in order for paint to adhere to the urethane. Cycle times are long for such a process, which adds to the cost of manufacture of each urethane part.
U.S. Pat. No. 3,726,952 describes the use of additives for polyurethane foam forming mixtures for the purpose of imparting mold release properties. The additives described contain at least 25 carbon atoms and are salts of an aliphatic carboxylic acid and either a primary amine or an amine containing amide or ester groups. While these additives may provide the release property to some extent, they have not been found to sufficiently reduce the forces necessary to release the molding from the mold quickly.
U.S. Pat. No. 3,725,105 describes the addition of silicones to epoxy coating compositions to impart a release surface to the coating after curing and thus prevent the adhesion of dirt and other foreign material thereto.
Lecithin has been employed for a variety of purposes. U.S. Pat. No. 3,314,814 discloses the use of lecithin as a bodying agent in coating compositions used in the manufacture of transfer papers. U.S. Pat. No. 3,649,541 describes the use of lecithin as a dispersing agent for ferromagnetic chromium oxide in coating compositions used to make magnetic tapes.
Lecithin has also found considerable use in the food industry. U.S. Pat. No. 3,060,030 describes its use as an emulsifier in baking mixes (cake mixes) where it prevents dipping or falling in the center of the cake and provides easy release from the pan after baking. Lecithin is also used as a dispersion in water with a propellant in an aerosol container for spraying onto cooking utensils to prevent the sticking of food during heating. This use is described in U.S. Pat. Nos. 3,661,605, 3,038,816 and 2,796,363.
U.S. Pat. Nos. 3,607,397 and 3,640,769 disclose the use of lecithin as an external mold release agent which is applied to the surface of the mold before charging a polyurethane-forming molding composition. They cite the fouling of the mold surfaces and prescribe methods for retarding such fouling or rejuvenating fouled surfaces. No instances have been found where lecithin has been used in polyurethane-forming molding compositions to impart self-release characteristics to such compositions.
Phosphorus compounds have been used as additives in polyester polyurethanes to prevent scorching or impart fire resistance. Examples of such uses are U.S. Pat. No. 3,429,837 and British Pat. No. 1,056,360. No prior art has been found, however, which teaches the use of lecithin or the phosphorus compounds specified herein to impart self-release properties to polyether polyurethanes.